Considerable literature is available describing slurry reactors for industrial, municipal, agricultural and farm solid waste and/or biomass digestion. The biomass is mixed with water or effluent to at least partially suspend the solid particles for intimate contact with the microorganisms. To further increase suspension, mixing and contacting, a gas, such as biogas circulation, for example, may be added to the reactor vessel.
Outside the field of biomass conversion, the majority of anaerobic water treatments make use of the principle of “Upflow Anaerobic Sludge Blanket” (UASB), introduced in the 1970s and updated ever since. The principle is outlined in EP-A 193 999 and EP-A 244 029. The characteristic of the UASB reactor is that the water to be treated is fed in and distributed over the bottom of a tank, from where it flows slowly upwards through a blanket of bacterial biomass. The gas thus produced bubbles upwards and provides for a certain degree of mixing. As a result of clever positioning of gas collection hoods below the water surface, the gas bubbles do not reach the water surface, with the result that a calm zone is produced at the top and any sludge particles swirled up are able to settle into the blanket of bacterial biomass (the “sludge blanket”) again.
In the art methane production from organic waste or biomass is often performed in two steps, acidogenesis and methanogenesis. In the acid digestion phase, volatile fatty acids (VFA), in particular acetic, propionic and butyric acid, are formed. In the scientific literature, a review is provided by Demirel Burak et al., Journal of Chemical Technology and Biotechnology, Vol. 77, No. 7, pp 743-755. In all processes, the acidogenic and methanogenic phases are kept physically separated. Some focus on VFA production, see e.g. Blasig J D et al., Resources, Conservation and Recycling, vol. 7, no. 1/03, pp 95-114. It teaches a continuous fermentation system with different removal rates for solids and liquids. As such, methanogenesis is considered disadvantageous and is avoided by manipulating fermentation conditions such as pH, temperature, and residence time.
Others attempt to improve the separate methanogenesis step: Xu H L et al., Water Science and Technology, Vol. 47, No. 1, 2003, pp 319-324, describe a coupled solid/liquid bioreactor, comprising a percolation reactor and an upward-flow anaerobic sludge blanket (UASB) reactor for the anaerobic digestion of food waste. Lepisto R et al., Water Research, Elsevier, Amsterdam, Vol. 33, No. 14, pp 3162-3170 focus on parameters influencing methanogenesis in UASB reactors. Wang Y et al., Biomass and Bioenergy, Vol. 33, No. 5, pp 848-853 also aim to control the volatile fatty acids concentrations in the methanogenic phase, in a single reactor concept.
The patent literature is no different. U.S. Pat. No. 4,022,665, EP 1,181,252 and U.S. Pat. No. 7,309,435 all describe methane production processes wherein the methane-forming fermentation phase is preceded by a separate acid digestion phase. The contents of these patent publications are hereby incorporated by reference.
In the art, these two-stage fermentors have been considered more effective than the conventional single-stage systems in the conversion of solid substrates to biogas. The reason rests in the fact that VFA production in concentrated feeds often proceeds at much faster rates than the subsequent conversion of VFA to methane, thereby causing acid accumulation and consequently inhibition of methanogenesis. A two-stage configuration was introduced to avert the imbalance between acidogenesis (i.e. VFA production) and methanogenesis by physically isolating these two major microbial phases in two separate bioreactors. VFA accumulation in the first reactor is often controlled by feeding—large—amounts of alkaline solution to said reactor. However, the use of additional streams of chemicals is often undesired. Also, both kinetic and thermodynamic biomass conversion rates are limited in these processes.
Hence, there is a continuous need in the art to improve the methanogenesis, in terms of yield [liter CH4 per kg of biomass] and kinetics [liter CH4 per m3 reactor per day].